With the window in its normal un-maximized size, there is no "Convert" button visible.
The buttons go directly from Preview to Options.
(See screenshot1)
The "Use Source Folder" button and text overlay the "Output Folder" in "Output Details"
(See screenshot2)
I just received bug 1733459 in Ubuntu:
For Lubuntu Next 17.10:
With the window in its normal un-maximized size, there is no "Convert" button visible. The buttons go directly from Preview to Options. (See screenshot1)
The "Use Source Folder" button and text overlay the "Output Folder" in "Output Details" (See screenshot2)